1. Field of the Invention
The present invention relates to a starting device for an air bag, and more particularly to a starting device which expands an air bag mounted to a vehicle to protect an occupant.
2. Description of the Related Art
An air bag is provided with a starting device for an air bag apparatus of a type which is mounted to a steering wheel, for example, this starting device expands the air bag when a vehicle rapidly decelerates or the like. The expanded air bag is then disposed between the steering wheel and an occupant.
for a starting device of this type, a drive shaft, which is rotatably provided within a case for the starting device, is rotated in accordance with an amount of inertial movement of a mass body upon a rapid deceleration of the vehicle. (A ball is generally used for the mass body.) Engagement between an engaging member (i.e., a triggering portion) provided at the drive shaft and an ignition pin is either released or unreleased. A mechanical starting device disclosed in Japanese Patent Application Laid-Open Nos. 60-248454, 60-248455, 60-248456, and 60-248457 is relatively popular. In accordance with the above-described released and unreleased states, the starting device switches from a state of blocking movement of the ignition pin urged in the direction of a detonator to a state in which the starting device does not block movement of the ignition pin.
for a starting device of this type, a circular arc portion is formed at a tip end portion of the engaging member of the drive shaft. The tip end portion is provided in the vicinity of a portion engaging the ignition pin. The dimensional accuracy of radius R of this circular arc must be extremely high so that the drive shaft can rotate smoothly at a predetermined timing when a predetermined inertial force is applied to a ball upon rapid deceleration of the vehicle.
According to the construction of the above-described prior art, an engaging member provided at the drive shaft and the ignition pin are engaged through a stepped collar portion provided at the ignition pin. When the movement of the ignition pin is blocked, the tip end portion of the engaging member remains in pressing contact against an outer peripheral surface of the stepped collar portion. As a result, the tip end portion becomes worn and changes shape by the vibrations or the like of the vehicle during a normal operation. Consequently, there is a possibility of failing to maintain the accuracy of the above-described radius R.
To solve this problem and to strengthen abrasion resistance of the tip end portion of the engaging member, it was necessary to take some special measures, such as abrasion resistance treatment and abrasion resistance machining. Such treatment and machining, however, increased the cost of a starting device.